1. Field of the Invention
The present invention relates generally to a continuous belt caster for casting thin cast block. More specifically, the invention relates to a side wall construction of a continuous belt caster.
2. Description of the Background Art
In recent years, various continuous belt casters have been developed and proposed for effectively casting relatively thin and continuous cast blocks. One of such continuous belt caster is a synchronous belt caster which defines a funnel-like path having a wider inlet and a narrower outlet so as to form solidified shell during travel therethrough. In general, such a synchronous belt caster has a pair of endless belts forming a moving wall of the caster and a pair of stationary side walls for defining the aforementioned funnel-like path. Each of the side walls has a wider transverse width at the portion in the vicinity of the inlet and a narrower transverse width at the portion in the vicinity of the outlet so as to define the funnel-like path gradually narrowing the path area toward the outlet.
Molten metal, such as molten pig iron or molten steel, is supplied to such belt caster through the inlet and cooled by transferring heat between the belts for gradually forming and gradually growing a solidified shell in the caster. The cast block is fed out or withdrawn through the outlet. During this process, the thickness of the solidified shell is reduced at a predetermined reduction rate by an essentially funnel shaped path. In such continuous casting process, it is desirable to maintain the melt in the vicinity of the side wall in a liquidus state. For this purpose, Japanese Patent First (unexamined) Publication Tokkai) Showa 58-218360 proposes a side wall construction provided with a refractory at the transverse center thereof. In the proposed construction, the refractory is supported on a metallic side wall body. The side wall body has positions extending along the side edge of the refractory and establishes tight contact with the endless belt.
Despite the presence of the refractory, it is still difficult to provide satisfactory delay of solidification. Namely, when the temperature of the melt is relatively low or when the speed of the melt flowing though the path is relatively low, substantial heat exchange is caused between the melt and the refractory for causing growth of the solidified shell on the refractory. On such an occasion, the solidified shell grown on the refractory is drawn together with the shells grown on the belt or the metallic edge portion of the side wall. If the stiffness of the solidified shell on the refractory is relatively low, it will still be possible to compress the shell during travel through the path toward the outlet. However, if the stiffness of the shell is substantial in such an extent that can resist against compression force exerted by the walls of the caster, since the solidified shell forms a wedge shaped block, metal penetration can be caused. If metal penetration occurs, movement of the solidified shell at the portion in the vicinity of the side wall can be completely prevented and movement of the solidified shell at the transverse central portion where the solidifying block mates with the belt is permitted to move. This tends to cause break-out of the cast block. In the alternative, because of excessive thickness of the shell, the belt will be subjected to substantial bending stress to cause damaging of the belt.
As a material to form the refractory layer on the side wall, one of silica, boron nitride, sialon and so forth can be used. Such materials generally have high heat conductivity to cause a greater magnitude of heat transfer between the metal to promote growth of the solidified shell when such material is solely used for forming the refractory layer. In addition, these materials as the refractory have relatively large linear expansion coefficients. Therefore, deformation can be caused in the metallic side wall body when substantial thermal expansion of the refractory is caused. On the other hand, when a material having low heat transfer coefficient, such as molten silica brick, is used for forming the refractory layer on the side wall, a solidified layer can grow not at the metallic side edge portion but also on the refractory layer. The solidified shell on the refractory layer tends to cause wearing of the surface of the refractory layer. Therefore, the refractory layer formed of the material having low heat transfer coefficient would not be applicable for the belt caster for long periods of use.
In order to obtain satisfactory delay in solidification of the melt in the portion in the vicinity of the refractory of the side wall, Japanese Patent First (unexamined) Publication (Tokkai) Showa 58-218326 discloses a technology of positively heating the refractory so as to prevent the melt from solidifying. However, this clearly increases costs and requires satisfactory isolation of electricity used for heating.